--- a/testing/marionette/driver.js
+++ b/testing/marionette/driver.js
@@ -2216,27 +2216,28 @@ GeckoDriver.prototype.sessionTearDown = 
   if (this.mainFrame) {
     try {
       this.mainFrame.focus();
     } catch (e) {
       this.mainFrame = null;
     }
   }
 
-  this.sessionId = null;
-
   if (this.observing !== null) {
     for (let topic in this.observing) {
       Services.obs.removeObserver(this.observing[topic], topic);
     }
     this.observing = null;
   }
 
   this.sandboxes.clear();
   cert.uninstallOverride();
+
+  this.sessionId = null;
+  this.sessionCapabilities = new session.Capabilities();
 };
 
 /**
  * Processes the "deleteSession" request from the client by tearing down
  * the session and responding "ok".
  */
 GeckoDriver.prototype.deleteSession = function (cmd, resp) {
   this.sessionTearDown();